File: /usr/src/linux/arch/arm/kernel/semaphore.c
1 /*
2 * ARM semaphore implementation, taken from
3 *
4 * i386 semaphore implementation.
5 *
6 * (C) Copyright 1999 Linus Torvalds
7 *
8 * Modified for ARM by Russell King
9 *
10 * This program is free software; you can redistribute it and/or modify
11 * it under the terms of the GNU General Public License version 2 as
12 * published by the Free Software Foundation.
13 */
14 #include <linux/config.h>
15 #include <linux/sched.h>
16
17 #include <asm/semaphore.h>
18
19 /*
20 * Semaphores are implemented using a two-way counter:
21 * The "count" variable is decremented for each process
22 * that tries to acquire the semaphore, while the "sleeping"
23 * variable is a count of such acquires.
24 *
25 * Notably, the inline "up()" and "down()" functions can
26 * efficiently test if they need to do any extra work (up
27 * needs to do something only if count was negative before
28 * the increment operation.
29 *
30 * "sleeping" and the contention routine ordering is
31 * protected by the semaphore spinlock.
32 *
33 * Note that these functions are only called when there is
34 * contention on the lock, and as such all this is the
35 * "non-critical" part of the whole semaphore business. The
36 * critical part is the inline stuff in <asm/semaphore.h>
37 * where we want to avoid any extra jumps and calls.
38 */
39
40 /*
41 * Logic:
42 * - only on a boundary condition do we need to care. When we go
43 * from a negative count to a non-negative, we wake people up.
44 * - when we go from a non-negative count to a negative do we
45 * (a) synchronize with the "sleeper" count and (b) make sure
46 * that we're on the wakeup list before we synchronize so that
47 * we cannot lose wakeup events.
48 */
49
50 void __up(struct semaphore *sem)
51 {
52 wake_up(&sem->wait);
53 }
54
55 static spinlock_t semaphore_lock = SPIN_LOCK_UNLOCKED;
56
57 void __down(struct semaphore * sem)
58 {
59 struct task_struct *tsk = current;
60 DECLARE_WAITQUEUE(wait, tsk);
61 tsk->state = TASK_UNINTERRUPTIBLE;
62 add_wait_queue_exclusive(&sem->wait, &wait);
63
64 spin_lock_irq(&semaphore_lock);
65 sem->sleepers++;
66 for (;;) {
67 int sleepers = sem->sleepers;
68
69 /*
70 * Add "everybody else" into it. They aren't
71 * playing, because we own the spinlock.
72 */
73 if (!atomic_add_negative(sleepers - 1, &sem->count)) {
74 sem->sleepers = 0;
75 break;
76 }
77 sem->sleepers = 1; /* us - see -1 above */
78 spin_unlock_irq(&semaphore_lock);
79
80 schedule();
81 tsk->state = TASK_UNINTERRUPTIBLE;
82 spin_lock_irq(&semaphore_lock);
83 }
84 spin_unlock_irq(&semaphore_lock);
85 remove_wait_queue(&sem->wait, &wait);
86 tsk->state = TASK_RUNNING;
87 wake_up(&sem->wait);
88 }
89
90 int __down_interruptible(struct semaphore * sem)
91 {
92 int retval = 0;
93 struct task_struct *tsk = current;
94 DECLARE_WAITQUEUE(wait, tsk);
95 tsk->state = TASK_INTERRUPTIBLE;
96 add_wait_queue_exclusive(&sem->wait, &wait);
97
98 spin_lock_irq(&semaphore_lock);
99 sem->sleepers ++;
100 for (;;) {
101 int sleepers = sem->sleepers;
102
103 /*
104 * With signals pending, this turns into
105 * the trylock failure case - we won't be
106 * sleeping, and we* can't get the lock as
107 * it has contention. Just correct the count
108 * and exit.
109 */
110 if (signal_pending(current)) {
111 retval = -EINTR;
112 sem->sleepers = 0;
113 atomic_add(sleepers, &sem->count);
114 break;
115 }
116
117 /*
118 * Add "everybody else" into it. They aren't
119 * playing, because we own the spinlock. The
120 * "-1" is because we're still hoping to get
121 * the lock.
122 */
123 if (!atomic_add_negative(sleepers - 1, &sem->count)) {
124 sem->sleepers = 0;
125 break;
126 }
127 sem->sleepers = 1; /* us - see -1 above */
128 spin_unlock_irq(&semaphore_lock);
129
130 schedule();
131 tsk->state = TASK_INTERRUPTIBLE;
132 spin_lock_irq(&semaphore_lock);
133 }
134 spin_unlock_irq(&semaphore_lock);
135 tsk->state = TASK_RUNNING;
136 remove_wait_queue(&sem->wait, &wait);
137 wake_up(&sem->wait);
138 return retval;
139 }
140
141 /*
142 * Trylock failed - make sure we correct for
143 * having decremented the count.
144 *
145 * We could have done the trylock with a
146 * single "cmpxchg" without failure cases,
147 * but then it wouldn't work on a 386.
148 */
149 int __down_trylock(struct semaphore * sem)
150 {
151 int sleepers;
152 unsigned long flags;
153
154 spin_lock_irqsave(&semaphore_lock, flags);
155 sleepers = sem->sleepers + 1;
156 sem->sleepers = 0;
157
158 /*
159 * Add "everybody else" and us into it. They aren't
160 * playing, because we own the spinlock.
161 */
162 if (!atomic_add_negative(sleepers, &sem->count))
163 wake_up(&sem->wait);
164
165 spin_unlock_irqrestore(&semaphore_lock, flags);
166 return 1;
167 }
168
169 /*
170 * The semaphore operations have a special calling sequence that
171 * allow us to do a simpler in-line version of them. These routines
172 * need to convert that sequence back into the C sequence when
173 * there is contention on the semaphore.
174 *
175 * ip contains the semaphore pointer on entry. Save the C-clobbered
176 * registers (r0 to r3 and lr), but not ip, as we use it as a return
177 * value in some cases..
178 */
179 #ifdef CONFIG_CPU_26
180 asm(" .section .text.lock, \"ax\"
181 .align 5
182 .globl __down_failed
183 __down_failed:
184 stmfd sp!, {r0 - r3, lr}
185 mov r0, ip
186 bl __down
187 ldmfd sp!, {r0 - r3, pc}^
188
189 .align 5
190 .globl __down_interruptible_failed
191 __down_interruptible_failed:
192 stmfd sp!, {r0 - r3, lr}
193 mov r0, ip
194 bl __down_interruptible
195 mov ip, r0
196 ldmfd sp!, {r0 - r3, pc}^
197
198 .align 5
199 .globl __down_trylock_failed
200 __down_trylock_failed:
201 stmfd sp!, {r0 - r3, lr}
202 mov r0, ip
203 bl __down_trylock
204 mov ip, r0
205 ldmfd sp!, {r0 - r3, pc}^
206
207 .align 5
208 .globl __up_wakeup
209 __up_wakeup:
210 stmfd sp!, {r0 - r3, lr}
211 mov r0, ip
212 bl __up
213 ldmfd sp!, {r0 - r3, pc}^
214
215 .previous
216 ");
217
218 #else
219 /* 32 bit version */
220 asm(" .section .text.lock, \"ax\"
221 .align 5
222 .globl __down_failed
223 __down_failed:
224 stmfd sp!, {r0 - r3, lr}
225 mov r0, ip
226 bl __down
227 ldmfd sp!, {r0 - r3, pc}
228
229 .align 5
230 .globl __down_interruptible_failed
231 __down_interruptible_failed:
232 stmfd sp!, {r0 - r3, lr}
233 mov r0, ip
234 bl __down_interruptible
235 mov ip, r0
236 ldmfd sp!, {r0 - r3, pc}
237
238 .align 5
239 .globl __down_trylock_failed
240 __down_trylock_failed:
241 stmfd sp!, {r0 - r3, lr}
242 mov r0, ip
243 bl __down_trylock
244 mov ip, r0
245 ldmfd sp!, {r0 - r3, pc}
246
247 .align 5
248 .globl __up_wakeup
249 __up_wakeup:
250 stmfd sp!, {r0 - r3, lr}
251 mov r0, ip
252 bl __up
253 ldmfd sp!, {r0 - r3, pc}
254
255 .previous
256 ");
257
258 #endif
259